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Org manual states in its beginning

Org is implemented on top of Outline mode, which makes it possible to keep the content of large files well structured.

I referenced 'outline-mode' by C-h f outline-mode, and curious about :

Is outline-mode is part of org, which was written by org team?
or it a emacs-builtin before Org?

How could I check it?

AbstProcDo
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1 Answers1

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One option is to check the source.

M-x find-library outline

You'll find in the header comments:

;; Copyright (C) 1986, 1993-1995, 1997, 2000-2019 Free Software Foundation, Inc.

;; This file is part of GNU Emacs.

M-x find-library org

;; Copyright (C) 2004-2019 Free Software Foundation, Inc.

;; This file is part of GNU Emacs.

So, both are included with Emacs but outline-mode predates org-mode.

Poking further you can see that org-mode is actually implemented as a derived mode of outline:

(define-derived-mode org-mode outline-mode "Org" ...

Community
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glucas
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    Another important point: outline is a single file with ~1000 lines of code. org is > 150000 lines of code in 200 files. So org started as an extension of outline, but the outline part is a very small piece of the whole system. – Tyler Jan 14 '20 at 17:12